The present invention relates to ion beam uniformity, particularly to assessing the uniformity of a raster-scanned ion beam, and more particularly to a high-sensitivity charge amplifier with bias compensation utilizing Faraday cups for measuring accumulated dose, or charge, in Coulombs, whereby an ion beam uniformity assessment can be made.
Raster-scanned ion beams have been utilized for various applications, such as for ion implantation. It is desirable to assess the uniformity of a raster-scanned ion beam to assure that the desired charge is uniform throughout the target area. Commercial instruments are available to measure the charge, as exemplified by Model MP-200 Uniformity Monitor, and Model MP-100 Dose Processor, each made by Varian/Extrion Division. However, these prior instruments have the following shortcoming: (1) not sensitive enough for low-dosage operation where the average current is in the femto-amp range, and (2) their integration technique is inaccurate for arbitrary waveforms. Recently, a Faraday cup array has been developed for increasing the sensitivity range and making the operative scaleable by use of an insertable plate in the aperture, as described and claimed in U.S. application Ser. No. 09/280,231, filed Mar. 29, 1999, entitled "Versatile, High-sensitivity Faraday Cup Array For Ion Implanters", and assigned to the same assignee.
The present invention addresses the needs of low-dose, low duty-cycle pulsed ion current waveforms. The invention uses a simple operational amplifier integrator with a bias current cancellation circuit to reduce drift, resulting in a high-performance, low-cost solution. The present invention utilizes four Faraday cups located in geometric corners of the ion beam target area, with current from each cup being integrated with respect to time, thus measuring accumulated dose, or charge, in Coulombs. By comparing the dose at each corner, a qualitative assessment of ion beam uniformity can be made.